A peak of a communication signal represents the greatest instantaneous amplitude, magnitude, or power level exhibited by a communication signal within some period of time. An average of the communication signal represents the average amplitude, magnitude, or power level of the communication signal over that same period. The peak is greater than the average, and the ratio of the peak power to the average power (PAPR) is a parameter of interest to communication system designers.
A successful communication system should maintain an adequately high signal-to-noise ratio (SNR) in signals received at receiving units. An adequately high SNR may be achieved by transmitting a signal from a transmitting unit at a high enough average power so that it is received at a receiving unit in excess of background noise power. Peak power is of less importance for these purposes. Specifically, the average power of the signal received at the receiving unit should exceed the background noise power by at least that SNR required to achieve acceptable quality data transfer over the link. And, any noise included in the transmitted signal broadcast from the transmitting unit should be low enough so that, when received at the receiving unit with the background noise, it leads to negligible increase in the total noise.
A successful communication system also keeps component costs low and uses power efficiently. One area which has a large impact on costs and power efficiency is the radio-frequency (RF) power amplifier included in transmitting units. Many modern modulation formats use communication signal amplitude, at least in part, to convey data. This amplitude modulation makes the use of a linear RF power amplifier desirable. But linearity is achieved only so long as the instantaneous amplitude of a communication signal remains beneath some maximum level. If the communication signal's peak power exceeds this maximum level, nonlinear amplification results, causing the spectrum of the communication signal to grow and exceed regulatory limitations imposed on the transmitting unit and also causing increased noise. Accordingly, the communication signal's peak power should be kept below this maximum level. But this maximum level should also be designed to be as low as possible to keep costs down. Significant costs are typically involved in providing power amplifiers and power amplifier biasing systems which support linear operation up to high peak amplitude levels.
Moreover, most linear power amplifiers become more power efficient as the PAPR decreases. Power amplifiers that accommodate high peak amplitude but have an average level far below this peak amplitude typically consume more power than would be needed to transmit the same average power level with a lower PAPR. Since many transmitting units are battery operated, the consumption of excessive power is a particularly undesirable design feature because excessive power consumption leads to the use of undesirably large batteries and/or frequent battery recharging.
Accordingly, many communication systems benefit from some sort of PAPR reduction prior to amplification in their transmitting units. Some benefits come in the form of reduced costs and power efficiency improvements in connection with providing and operating RF power amplifiers. And, other benefits come from operating transmitting units at a greater average power, which increases link margins and permits greater amounts of data to be transmitted in a given period of time. As a general rule, a small amount of peak reduction leads to only a small amount of benefit, and greater benefit results from greater amounts of peak reduction.
But certain constraints limit the amount of peak reduction that may be achieved. One such constraint is a maximum limit on the amount of noise included in the transmitted signal broadcast from the transmitting unit. This constraint may be designated as an error vector magnitude (EVM) specification. EVM specifications are based upon achieving a desired SNR at a receiving unit for a given modulation order and coding rate. EVM may be designated as the ratio of the total amount of noise power in a communication signal to the total signal power in that signal. It is usually specified as a percentage, equal to one-hundred divided by the square-root of the SNR.
Another constraint is imposed by governmental regulations which limit the spectral emissions from a transmission unit. Such regulations may be referred to as a spectral mask. A typical spectral mask permits a maximum average in-band power level to be emitted from a transmitting unit within a specified bandwidth. But outside that bandwidth the emitted power is severely restricted. A certain amount of out-of-band power, usually far less than the maximum average in-band power, is typically permitted in the portion of the spectrum adjacent to the specified bandwidth, with further diminishing amounts of out-of-band power being permitted farther from the specified bandwidth.
These constraints have caused conventional PAPR reduction techniques to be less effective than they might have been, causing conventional communication systems to obtain less peak-reduction benefit than desired. And, as spectral mask constraints become more stringent, the conventional PAPR techniques are even less effective.
One conventional technique passes a communication signal, which otherwise includes little or no noise and meets its spectral mask constraints, through a hard limiter, clipping off that portion of the communication signal that exceeds a threshold. The clipping function causes the clipped communication signal to differ from its ideal shape, infusing in-band and out-of-band noise into the clipped communication signal. The out-of-band noise resulting from the clipping function also causes spectral regrowth in excess of the amounts permitted by its spectral mask. But spectral mask compliance is reestablished by filtering the clipped communication signal to reduce the out-of-band noise beneath the amount specified by the spectral mask. And, the clipping threshold is adjusted to be as high as it needs to be so that the clipped communication signal with the remaining in-band noise meets EVM specifications.
This clip-and-filter technique suffers from two problems. First, thresholds are usually set relatively high in order to meet EVM specifications. Consequently, only a small amount of peak-reduction and peak-reduction benefit is achieved. Second, the in-band noise introduced by the clipping function is infused into the clipped communication signal itself so that it cannot be processed separately from the communication signal. In modulation formats where multiple carriers or channels have been combined in the communication signal, different EVM specifications may apply to the different carriers or channels. But after the in-band noise has been infused into the communication signal the noise portion cannot be processed without applying the same processing to the communication signal itself. Consequently, all carriers or channels are forced to comply with the most stringent EVM specification for the different modulation formats being conveyed.
This problem of immediately infusing in-band noise into the communication signal may be addressed in an alternate technique by performing an excursion generation function, which is nearly opposite to the function performed by the hard limiter of the clip-and-filter technique. The excursion generation function substitutes zero magnitude samples for all samples in the communication signal less than the threshold, and passes all samples having a magnitude greater than or equal to threshold, but reduced in magnitude by the magnitude of the threshold. Thus, a resulting excursion signal conveys only the portion of the communication signal that exceeds the threshold. This excursion signal may then be processed to control the in-band noise being applied to different carriers or channels without disturbing the communication signal with such processing. Then, after such processing, the excursion signal is recombined with the original communication signal to cancel peak events. But the excursion generation function also introduces out-of-band noise that is filtered out to meet spectral mask requirements. So, the processed cancellation signal consists almost entirely of in-band noise. As with the clip-and-filter technique, thresholds are set higher than desired in order to meet EVM specifications. Consequently, less peak-reduction and less peak-reduction benefit are achieved than is desired.
Both of the above discussed peak reduction techniques rely almost exclusively on in-band noise to reduce peaks. Out-of-band noise is permitted in the peak reduction efforts only to the extent that it remains beneath a spectral mask. In other words, so little out-of-band energy is used for peak reduction that out-of-band energy asserts virtually no influence in the peak-reduction process.
Other techniques have been discussed which rely exclusively on out-of-band nulling waveforms or artifacts to limit peaks. The out-of-band nulling waveforms or artifacts are added to a communication signal to limit peaks, and then filtered off after amplification in an RF power amplifier. In these techniques, no clipping or excursion generation functions are performed. Consequently, in-band noise from clipping or excursion generation functions is not introduced into the communication signal and EVM does not suffer from peak-reduction efforts. Unfortunately, very little peak reduction may be achieved relying exclusively on the addition of out-of-band nulling waveforms or artifacts to a communication signal. Consequently, in spite of avoiding functions that worsen EVM, less peak-reduction and less peak-reduction benefit are achieved than is desired.